Yesterday, I downloaded the latest version of MiKTeX and Texmaker and started using XeLaTeX. Today, I am no longer able to compile or save any .tex file. When I try to do so, a message pops up: “The file could not be saved. Please check if you have write permission.”

With TeXmaker 4.0.1, save it as .tex extension otherwise it will pop up error. It is not saved as .tex automatically starting from new version as latex file can have any extensions .cls.sty.dtx etc..
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texenthusiastMar 30 '13 at 22:30

Which windows? Did you install as a admin and work now as a non-admin (and the tex-file is in a system folder)?
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knutMar 30 '13 at 22:38

2

Ok, I saved the file as .tex and now it works fine!
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benMar 30 '13 at 22:50

2

Not too localized at all – this is easily reproducible and will likely annoy many users; yet the author of Texmaker is unwilling to change it. (Voted to reopen.)
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doncherryApr 11 '13 at 5:38

@ben I added the error message I’m getting, assuming you’re getting the same one? This way, people Googling that message will hopefully find this question.
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doncherryApr 11 '13 at 16:27